External fixation systems are used to stabilize fractured bones or to secure bones after corrective surgery. They are usually made up of structural members held together by clamps, all assembled by the surgeon during surgery. The clamps are placed on bone pins and are attached to bars, creating a frame to hold the bones in particular relationships. Typically, the external fixation frame is assembled in the configuration the surgeon desires, then the fracture is reduced and the clamps are tightened. Some conventional clamps have to be tightened partially to provisionally lock the bone pin or bar into the clamp.
In one known external fixation clamping apparatus, the fixation element is inserted against an axial spring force that forces the outer and inner jaws together. The same spring provides a provisional locking force that retains the fixation element prior to final clamping. The force of the spring is dependent on the degree to which a nut is threaded into the clamping apparatus. In the fully unthreaded state, the force is relatively low and the fixation element can be inserted rather easily. It can pop out rather easily as well. As the nut is progressively threaded into the clamping apparatus, this spring force becomes greater and at some point the clamping apparatus is threaded to such a degree that the insertion of a fixation element is mechanically prohibited. The point at which this occurs also varies based on whether the fixation element is the first or the second element to be inserted into the clamping apparatus. It is therefore possible to find that after insertion of the first fixation element, the insertion of the second fixation element requires that the clamping apparatus be loosened. This variability is, at a minimum, a nuisance. In other known devices, the spring force acts on a latch slide that must be forced open by the act of inserting the fixation element. The spring element acted on during the act of insertion only provides a return force on the latch device once the fixation element is inserted; it does not provide any provisional clamping force and so the fixation element, while retained, is loose in the clamp prior to tightening the clamping element. This too is, at a minimum, a nuisance.